In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, mobile terminals, wireless devices, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks, which provide access to data networks, such as the Internet, and/or the public-switched telecommunications network (PSTN). The RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station (also referred to as a network node, base station, a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband data services based on, for example, WCDMA (Wideband Code Division Multiple Access), HSPA (High Speed Packet Access), and Long Term Evolution (LTE) wireless technologies. Moreover, fuelled by introduction of new devices designed for data applications, end user performance requirements are steadily increasing. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by high-speed wireless data networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently are desired.
Techniques to improve downlink performance may include 4-branch Multiple-Input Multiple-Output (MIMO), multi-flow communication, multi-carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for wireless networks may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell.
A homogeneous network is a network of base stations in a planned layout, providing communications services for a collection of wireless terminals in which all base stations may have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may offer unrestricted access to wireless terminals in the network, and each base station may serve roughly a same number of wireless terminals. Current cellular wireless communications systems in this category may include, for example, Global System for Mobile communication (GSM), WCDMA, High Speed Downlink Packet Access (HSDPA), LTE, Worldwide Interoperability for Microwave Access (WiMAX), etc.
In a heterogeneous network, low power base stations (also referred to as low power nodes, LPNs, micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit nodes, RRU nodes, small cells, RRUs, etc.) may be deployed along with or as an overlay to planned and/or regularly placed macro base stations. A macro base station (MBS) may thus provide service over a relatively large macro cell area and each LPN may provide service for a respective relatively small LPN cell area within the relatively large macro cell area. Power transmitted by an LPN (e.g., 2 Watts) may be relatively small compared to power transmitted by a macro base station (e.g., 40 Watts for a typical macro base station). An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in the coverage provided by the macro base stations, and/or to off-load traffic from macro base stations (e.g., to increase capacity in a high traffic location, also referred to as a hot-spot). Due to the lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition.
In initial discussions among members of the 3rd-Generation Partnership Project (3GPP) regarding the development of Release 12 specifications for LTE, one of the proposed items for study is the possibility of simultaneously serving a UE from more than one eNB, referred to as “dual connectivity.” The control and user plane procedures of LTE may be updated to support dual connectivity.